In software engineering, a web application is an application that is accessed via a web browser over a network such as the Internet or an intranet. Web applications are popular due to the ubiquity of a client, sometimes called a thin client, such as a web browser. A significant advantage of building web applications to support standard browser features is that they can perform as specified regardless of the operating system (OS) or OS version installed on a given client. Rather than creating clients for Microsoft Windows, Mac OS X, GNU/Linux, and other operating systems, the application developer can write the application once and deploy it almost anywhere. The ability to update and maintain web applications without distributing and installing software on potentially thousands of client computers is one reason for their popularity. Common web applications include webmail, online retail sales, online auctions, wikis, discussion boards, weblogs, massively multiplayer online role-playing games (MMORPGs), and many others.
The web interface places very few limits on client functionality. Through Java, JavaScript, Dynamic HyperText Markup Language (DHTML), Adobe Flash, and other technologies, application-specific methods such as drawing on the screen, playing audio, and access to the keyboard and mouse are all possible. Many services have worked to combine all of these into a more familiar interface that adopts the appearance of an operating system. General-purpose techniques such as drag and drop are also supported by these technologies. Web developers often use client-side scripting to add functionality, especially to create an interactive experience that does not involve page reloading (which many users find disruptive). Recently, technologies have been developed to coordinate client-side scripting with server-side technologies such as PHP. Ajax, a web development technique using a combination of various technologies, is an example of a technology that creates a more interactive experience.
Though many variations are possible, a web application is commonly structured as a three-tiered application. In its most common form, a web browser is the first tier, an engine using some dynamic web content technology (such as ASP, ASP.NET, CGI, ColdFusion, JSP/Java, PHP, embPerl, Python, or Ruby on Rails) is the middle tier, and a database or application server is the third tier. The web browser sends requests to the middle tier, which services the request by making queries against the application server. The web server also generates a web-based user interface provided to the client.
An emerging strategy for application software companies is to provide web access to software previously distributed as local applications. Depending on the type of application, it may involve the development of an entirely different browser-based interface, or merely adapting an existing application to use different presentation technology. These programs allow the user to pay a monthly or yearly fee for use of a software application without having to install it on a local hard drive. A company that follows this strategy is known as an application service provider (ASP), and ASPs are currently receiving much attention in the software industry.
In earlier types of client-server computing, each application had its own client program that served as its user interface and was separately installed on each user's personal computer. An upgrade to the server part of the application would typically involve an upgrade to the clients installed on each user workstation, adding to the support cost and decreasing productivity. In contrast, web applications dynamically generate a series of web documents in a standard format, such as HyperText Markup Language (HTML)/eXtended HTML (XHTML), supported by common browsers. Client-side scripting in a standard language such as JavaScript is commonly included to add dynamic elements to the user interface. Generally, each individual web page is delivered to the client as a static document, but the sequence of pages can provide an interactive experience, as user input is returned through web form elements embedded in the page markup. During the session, the web browser interprets and displays the pages, and acts as the universal client for any web application.
As noted above, the user interface experience of web applications is typically one of loading a page, submitting form elements, and then loading a subsequent page. Unlike traditional local applications, because of the slowness imposed by the network roundtrip in between the client and server, web applications are often designed to transmit as little data as possible. While this creates a faster experience by reducing page loads, it also unfortunately generally reduces the interactivity of the application. Application developers may even choose not to offer certain features through the web that are available in a client version of the application. For example, Project Server 2007 offers users some capabilities for editing their project plans on the web, however, the editing experience is limited. Users can make a series of edits to tasks properties, after which they have to manually hit a button for the project to be validated and scheduled. The entire project then goes through a queue to be scheduled (which can be slow depending on the complexity of the project and the number of other activities in the queue waiting to be processed). The lack of immediate feedback after changing a task's properties means that users will not know if they have made invalid or contradictory edits that cause errors until they have scheduled the entire project.